Melt-induced weakening can play critical role for enabling lithospheric
deformation in the areas of intense mantle-derived magmatism, such as mid-ocean
ridges, rift zones and hot spots. It implies significant reduction in the
long-term strength of the deforming lithosphere subjected to frequent rapid melt
percolation episodes along planar, sharply localized zones (dykes). Mechanical
energy dissipation balance shows that the long-term effective strength of the
melt-weakened lithosphere is a strain-averaged rather than a time-averaged
quantity. Its magnitude is mainly defined by the ratio between melt pressure and
lithostatic pressure along rapidly propagating dykes, which control most of the
visco-plastic lithospheric deformation. We implemented governing equations for
melt-bearing deforming visco-elasto-plastic lithosphere based on staggered
finite difference and marker in cell techniques. We then quantified the
lithospheric strength by performing 2D numerical experiments on long-term
lithospheric deformation assisted by frequent short-term dyke propagation
episodes. The experiments showed that the lithospheric strength can be as low as
few MPa and is critically dependent on the availability of mantle-derived melt
for enabling frequent episodes of dyke propagation. Viscous-plastic deformation
is localized along propagating weak dykes whereas bulk of the lithosphere only
deforms elastically and is subjected to large deviatoric stresses. Thus, the low
strength of the melt-weakened lithosphere is associated with high
volume-averaged deviatoric stress. Possible geodynamic implications include (1)
establishing of a global tectono-magmatic plume-lid tectonics regime in the
Archean Earth and modern Venus as well as (2) enabling of plume-induced
subduction initiation that triggered global modern-style plate tectonics on
Earth.

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